typedef struct DArray {
    int end;
    int max;
    size_t element_size;
    size_t expand_rate;
    void **contents;
} DArray;

typedef int (*DArray_compare) (const void *a, const void *b);

int compare(const void *a, const void *b)
{
  int i = (int)*a;
  int k = (int)*b;
  printf("%d %dn", i, k);
  return i - k;
}

error: operand of type 'void' where arithmetic or pointer type is required int i = (int)*a;

int i = *(int*)a;
// This one has more parens to make it really obvious what your intent is.
int k = *((int*)b);

// Cast b from a pointer to a void into a pointer to an int.
int *X = (int*)b;
// k = "what X is pointing to" or "the contents of X"
int k = *X;

typedef int (*DArray_compare) (const int *a, const int *b);

int compare(const int *a, const int *b)
{
    int i = *a;
    int k = *b;
    ...

int compare(const void *a, const void *b)
{
    const DArray *d1 = a;
    const DArray *d2 = b;

    if (d1->end < d2->end)
        return -1;
    else if (d1->end > d2->end)
        return +1;
    else if (d1->max < d2->max)
        return -1;
    else if (d2->max > d2->max)
        return +1;
    else
        return 0;
}

if (i < k)
    return -1;
else if (i > k)
    return +1;
else
    return 0;